Can't define a custom association (ST2-B3)

In our project we would like to create a custom association class that works a little bit different than the belongsTo association. Unfortunately this seemingly old piece of code in Ext.data.association is giving me a headache:

The only thing that I think you might run into is the fact that reading in associated data in Model.js is checking for the type on the association. If you make a custom one, this code would not be triggered.